1. Field of the Invention
The invention relates to the field of solid state electronics, and particularly, the invention relates to a method of fabricating an improved silicon controlled rectifier (SCR) in an integrated circuit (IC), and an improved SCR so formed.
2. Description of the Existing Art
In general, a silicon controlled rectifier (SCR) is a unidirectional semiconductor device which is primarily used for bistable switching. Such SCRs may be fabricated as discrete components or as part of an integrated circuit (IC). Specifically, an SCR is a four-layer device having three PN junctions. Three terminals, anode, cathode and gate, each conductively connects to a corresponding layer of the SCR.
With no voltage applied to the gate terminal, a typical SCR remains off for all values of forward voltage, i.e., positive anode-to-cathode voltage, below a certain voltage level (value) known as the breakover voltage. However, once the breakover voltage is exceeded by the forward voltage, the SCR conducts current from the anode terminal to the cathode terminal. Typically, this current, known as forward current, is limited by a limiting resistor connected in series with the SCR anode or cathode terminal. Once the SCR begins conducting current, a small forward voltage produces a large forward current, and this large forward current continues to flow through the SCR as long as the forward current does not fall below a minimum current known as the holding current. In contrast, when an SCR is reverse biased, i.e., when the anode-to-cathode voltage is negative and there is no gate voltage or the gate terminal is an open circuit, the SCR does not conduct current.
In general, a relationship exists between forward voltage and forward current of an SCR. This relationship varies as a positive voltage is applied to the gate terminal of the SCR. Specifically, as gate current increases due to an increasing positive voltage being applied to the gate terminal, the breakover voltage decreases to a lower value of forward voltage than that which occurs without a gate current present. Consequently, for any given value of gate current, a particular forward breakover voltage must be attained before the SCR will conduct. Therefore, SCR operation depends on both gate current and forward voltage. An SCR is said to have been "triggered" when sufficient gate current is generated to permit forward current to flow through the SCR, i.e., when the breakover voltage falls below an applied, fixed value of forward voltage.
In a typical application, a control circuit is connected to the SCR to regulate triggering. Some SCR control circuits include a shunt resistor which connects between the gate terminal and the cathode terminal. The shunt resistor is a variable resistor which controls triggering by regulating the amount of gate current. Typically, the shunt resistor valve is conventionally controlled by electrical or optical control signals being applied to the shunt resistor. For example, given an SCR with a fixed forward voltage, when the shunt resistor has a relatively low, e.g., tens of ohms, or zero resistance value, the shunt resistor "holds off" SCR triggering by essentially clamping the gate voltage to the cathode voltage. With the gate voltage clamped, the flow of gate current is zero or nearly zero and the SCR will not trigger. Consequently, forward current is also zero since, without gate current, the breakover voltage is a relatively high voltage value, i.e., the breakover voltage is presently larger than the forward voltage that will cause the SCR to conduct. However, when the shunt resistor has a relatively high resistance value, e.g., millions of ohms, SCR triggering occurs easily because gate current readily flows in response to relatively small positive gate voltages. Due to the flow of gate current, the breakover voltage decreases to a value below the applied forward voltage, thus, the SCR conducts forward current.
Although existing SCR control circuits have served the purpose, they have not proved entirely satisfactory for at least two reasons. First, adjustable shunt resistors that are used for trigger control in most existing SCR control circuits are fabricated as discrete devices external to the SCR. As such, apparatus using such circuits have been relatively expensive to fabricate, i.e., these circuits require many discrete components rather than a single integrated circuit. Second, those shunt resistors that are fabricated as an integrated part of the SCR are typically fixed value resistors and, consequently, they do not perform trigger-control functions. Such fixed value resistors simply establish a fixed value of gate current at which the SCR triggers. In these applications, the value of gate voltage controls SCR triggering. As such, triggering the SCR using optical radiation is not possible without additional componentry.
Thus, a need exists in the art for an improved SCR having an adjustable value shunt resistor integrated into an IC and a method of fabricating such an improved SCR.